1. Field of the Invention
The present invention relates to a phase change memory that can store information and electrically rewrite the information by using a material that has a current resistance value fluctuating with a structural change caused by a phase change in response to the passage of current through a device.
2. Description of the Related Art
Solid storages for recording data in NAND flash memories feature high-speed access, high data transfer rates, and low power consumption and thus have received attention as next-generation storage devices. Memory devices have been reduced in size in order to produce solid storages with larger capacities. It is expected that coupling between adjacent memory devices may saturate a storage density in the near future, requiring solid storages with high speeds and large capacities instead of NAND flash memories.
Resistive random access memories having been earnestly studied as next-generation solid storages include a phase change memory composed of a chalcogenide material that is a recording material. The basic structure of a memory cell, that is, a phase change memory device, contains a recording material between metal electrodes. A phase change memory is a resistive random access memory that stores information by using varying resistance states of a recording material between electrodes.
The phase change memory cell stores information according to the resistance value of a recording material composed of phase change materials such as Ge2Sb2Te5, the resistance value changing between an amorphous state and a crystalline state. The recording material has a high resistance in an amorphous state and has a low resistance in a crystalline state. Thus, information is read by determining a potential difference across a memory cell, measuring a current passing through the memory cell, and identifying whether the memory cell is in a high resistance state or a low resistance state.
In recent years, a theory of a phase change memory controllable only by transferring Ge atoms has been proposed. A phase change memory based on this theory is disclosed in “Interfacial phase-change memory”, Nature Nanotechnology Vol. 6 p. 501-505 (2011), R. E. Simpson and 6 others. The memory having a superlattice structure of alternately stacked GeTe and Sb2Te3 layers switches between a high-resistance crystalline state and a low-resistance crystalline state. The phase change memory having the superlattice structure can switch at a lower current than a conventional phase change memory composed of phase change materials such as Ge2Sb2Te5, thereby reducing power consumption.
A reduction in the operating current and power of a phase change memory cell is an important factor of technical development. If the operating current of the phase change memory cell can be reduced, switches for selecting the memory cells of, for example, MOS transistors and diodes can be reduced in size, thereby increasing the density and speed of a solid storage. Furthermore, if the operating power of the phase change memory cell can be reduced, a solid storage with the phase change memory cell is applied to a storage class memory for mobile and home PCs (a high-speed memory capable of compensating for a performance gap between a cash memory such as DRAM and an external storage to improve the performance of a device and reduce power consumption), thereby effectively reducing the power consumption of these devices. Rewriting of data (a resetting operation in particular) requires at least 60% of the operating current and power consumption of the phase change memory cell. Thus, it is important to reduce a current and power required for the resetting operation.
The resetting operation corresponds to an increase in resistance in the phase change memory cell. Thus, an increase in resistance in the superlattice phase change memory cell can achieve lower power.